I was thinking that same thing... Response to the OP: If u take away and put back the redstone signal do both arms respond? One will probably disappear if you do that, then glitch solved.
I deactivated the current and the bottom arm remained while the side arm detracted: I have a F2 but im on my laptop, destroying it got rid of the problem but now I have the 2 head problem with it looking like the piston has and hasn't extended and wont close again so the lava is breaking